Your iMac has support for adding an external monitor via the Thunderbolt port.
You will need an adapter and cable, depending on the connection available in the display - for example, if the display has HDMI connection, you need a Thunderbolt 2 to HDMI adapter, and an HDMI cable.
If it is at all possible to add a second external display, it will require additional effort.
Maybe there are some Thunderbolt docks with dual HDMI displays; that might work, but it will be expensive - such docks may not be easy to find, and may cost more than the display.
Another possibility, which I mention for completeness, but I cannot recommend and never tried: use some USB dock with DisplayLink software. The OS has never supported displays over traditional USB3 connections, but there are third party docks that support this. As noted, this depends on having a dock and third party software.
Given that this is now an old mac, approach this with tempered expectations, and consider carefully whether to spend money on gear for it. It may better to use one display now, and save towards a future mac.